4.8. What are the Overall Climate Effects of Subsonic Aircraft?

The climate impacts of different anthropogenic emissions can be compared
using the concept of radiative forcing. The best estimate of the radiative forcing
in 1992 by aircraft is 0.05 Wm-2 or about 3.5% of the total radiative forcing
by all anthropogenic activities. For the reference scenario (Fa1), the radiative
forcing by aircraft in 2050 is 0.19 Wm-2 or 5% of the radiative forcing in the
mid-range IS92a scenario (3.8 times the value in 1992). According to the range
of scenarios considered here, the forcing is projected to grow to 0.13 to 0.56
Wm-2 in 2050, which is a factor of 1.5 less to a factor of 3 greater than that
for Fa1 and from 2.6 to 11 times the value in 1992.

Figure 3: Estimates of the globally and annually
averaged total radiative forcing (without cirrus
clouds) associated with aviation emissions under each
of six scenarios for the growth of aviation over the
time period 1990 to 2050. (Fa2 has not been drawn
because the difference from scenario Fa1 would not
be discernible on the figure.)

These estimates of forcing combine the effects from changes in concentrations
of carbon dioxide, ozone, methane, water vapor, line-shaped contrails, and aerosols,
but do not include possible changes in cirrus clouds.

Globally averaged values of the radiative forcing from different components
in 1992 and in 2050 under the reference scenario (Fa1) are shown in Figure
2. Figure 2 indicates the best estimates
of the forcing for each component and the two-thirds uncertainty range.8 The derivation of these uncertainty ranges involves expert scientific judgment
and may also include objective statistical models. The uncertainty range in
the radiative forcing stated here combines the uncertainty in calculating the
atmospheric change to greenhouse gases and aerosols with that of calculating
radiative forcing. For additional cirrus clouds, only a range for the best estimate
is given; this is not included in the total radiative forcing.

The state of scientific understanding is evaluated for each component. This
is not the same as the confidence level expressed in previous IPCC documents.
This evaluation is separate from the uncertainty range and is a relative appraisal
of the scientific understanding for each component. The evaluation is based
on the amount of evidence available to support the best estimate and its uncertainty,
the degree of consensus in the scientific literature, and the scope of the analysis.
The total radiative forcing under each of the six scenarios for the growth of
aviation is shown in Figure 3 for the period 1990
to 2050.

The total radiative forcing due to aviation (without forcing from additional
cirrus) is likely to lie within the range from 0.01 to 0.1 Wm-2 in 1992, with
the largest uncertainties coming from contrails and methane. Hence the total
radiative forcing may be about 2 times larger or 5 times smaller than the best
estimate. For any scenario at 2050, the uncertainty range of radiative forcing
is slightly larger than for 1992, but the largest variations of projected radiative
forcing come from the range of scenarios.

Over the period from 1992 to 2050, the overall radiative forcing by aircraft
(excluding that from changes in cirrus clouds) for all scenarios in this report
is a factor of 2 to 4 larger than the forcing by aircraft carbon dioxide alone.
The overall radiative forcing for the sum of all human activities is estimated
to be at most a factor of 1.5 larger than that of carbon dioxide alone.

The emissions of NOx cause changes in methane and ozone, with influence on
radiative forcing estimated to be of similar magnitude but of opposite sign.
However, as noted above, the geographical distribution of the aircraft ozone
forcing is far more regional than that of the aircraft methane forcing.

The effect of aircraft on climate is superimposed on that caused by other anthropogenic
emissions of greenhouse gases and particles, and on the background natural variability.
The radiative forcing from aviation is about 3.5% of the total radiative forcing
in 1992. It has not been possible to separate the influence on global climate
change of aviation (or any other sector with similar radiative forcing) from
all other anthropogenic activities. Aircraft contribute to global change approximately
in proportion to their contribution to radiative forcing.

4.9. What are the Overall Effects of Subsonic Aircraft on UV-B?

Ozone, most of which resides in the stratosphere, provides a shield against
solar ultraviolet radiation. The erythemal dose rate, defined as UV irradiance
weighted according to how effectively it causes sunburn, is estimated to be
decreased by aircraft in 1992 by about 0.5% at 45�N in July. For comparison,
the calculated increase in the erythemal dose rate due to observed ozone depletion
is about 4% over the period 1970 to 1992 at 45�N in July.9 The net effect of subsonic aircraft appears to be an increase in column
ozone and a decrease in UV radiation, which is mainly due to aircraft NOx emissions.

Much smaller changes in UV radiation are associated with aircraft contrails,
aerosols, and induced cloudiness. In the Southern Hemisphere, the calculated
effects of aircraft emission on the erythemal dose rate are about a factor of
4 lower than for the Northern Hemisphere.

For the reference scenario (Fa1), the change in erythemal dose rate at 45�N
in July in 2050 compared to a simulation with no aircraft is -1.3% (with a two-thirds
uncertainty range from -0.7 to -2.6%).

For comparison, the calculated change in the erythemal dose rate due to changes
in the concentrations of trace species, other than those from aircraft, between
1970 to 2050 at 45�N is about -3%, a decrease that is the net result of two
opposing effects: (1) the incomplete recovery of stratospheric ozone to 1970
levels because of the persistence of long-lived halogen-containing compounds,
and (2) increases in projected surface emissions of shorter lived pollutants
that produce ozone in the troposphere.